1. Field of the Invention
The present invention relates to the field of MOS integrated amplifiers and more specifically to CMOS differential amplifiers.
2. Related Application
This application is related to a co-pending application, Ser. No. 207,668, filed June 16, 1988, and entitled "Self-Biased, High-Gain Differential Amplifier".
3. Prior Art
In the design of complementary metal-oxide semiconductor (CMOS) integrated circuits, differential amplifiers are used for various applications because a number of advantages can be derived from these types of amplifiers, as compared to single-ended amplifiers. Differential amplifiers are used to amplify analog, as well as digital signals, and can be used in various implementations to provide an output from the amplifier in response to differential inputs. For example, a general-purpose differential amplifier amplifies the difference of the two input signals. But these differential amplifiers can be readily adapted to function as an operational amplifier, a comparator, a sense amplifier and as a front-end buffer stage for another circuit. Differential amplifiers are utilized where linear amplification having a minimum of distortion is desired.
However, a typical differential amplifier will operate only over a relatively narrow range of common-mode input voltages. As the amplifier is forced to extend beyond this small range of common-mode voltages, the differential-mode gain drops off sharply and in some instances drops to zero.
One technique for improving the range of this common-mode input voltage range is described in "A Highly Linear CMOS Buffer Amplifier"; Fisher, John A.; IEEE Journal of Solid-State Circuits, Vol. sc-22, No. 3; pp. 330-334; June 1987. Although this paper describes improvements in linearity and drive capability over previous wide-input-range amplifiers, the biasing scheme it uses is uncompensated for variations in common-mode voltage, supply voltage, temperature, and process. Because of the lack of compensation, the practical common-mode range of the differential inputs is somewhat less than the full rail-to-rail range. Further, it relies on the use of saturated current sources for biasing the circuitry, and such biasing techniques can result in reduced common-mode range and amplifier bandwidth.
It is appreciated then that what is needed is a differential amplifier for amplifying differential input voltages in which the common-mode component can vary over an extremely wide range of voltages. Further, it is most desirable for that wide range of voltages to extend as far as the rail-to-rail voltage, while maintaining the differential-mode gain of the amplifier at a high level.